Field of the Invention
The present invention relates to a track access control circuit for an optical disk unit that carries out a seek operation by using a double servo, i.e., a control of the velocity of a movable part in an actuator incorporated in an optical head block and a control of the the position of a positioner driven by a Voice-Coil Motor (VCM).
In an optical disk unit, a track pitch can be set to be as small as several microns. Therefore, the storage capacity of the optical disk unit is huge. This is the reason why the optical disk unit is attracting attention in recent years for use as a large capacity storage device for a computer system and so forth.
The optical disk includes an optical head which has an actuator mounting an object lens. The actuator has a moving part for moving a light beam spot to a target position on the disk. A two-dimensional actuator is known as an example of the actuator. The actuator is mounted on a positioner driven by a voice-coil motor. Therefore, in the following, the positioner is also referred to as a VCM positioner. The actuator has a moving part of, for example, an arm and an object lens attached to an end of the arm in case of the two-dimensional actuator. In the two dimensional actuator, tracking control is carried out by turning the arm around a center shaft of the arm to move the lens. In this way, a light beam is moved across tracks on a disk medium, and focusing control is carried out by driving the arm along the shaft. The present invention relates to the tracking control.
To move the object lens to the target position, there are three kinds of controls, i.e., a coarse control, a lens-seek control, and a fine control. In the coarse control, the VCM positioner is controlled by the voice-coil motor (VCM) and the focusing control is effected on the two-dimensional actuator. The tracking control of the actuator is not effected in the coarse control. In the lens-seek control, the actuator is controlled to move the object lens over 20 to 30 tracks to the target position. Independent from the lens-seek control, the positioner is controlled by a positioner servo circuit so as to keep the position of the actuator at the neutral position with respect to the positioner. Namely, the control of the VCM positioner and the control of the actuator are carried out by separate servo loops which are called as double servo. In the fine control, the tracking control is effected on the object lens rotatably mounted on the two-dimensional actuator and the VCM positioner is also controlled to follow the movement of the object lens.
In the coarse control, the two-dimensional actuator can access a track by moving the positioner within a relatively wide range of tracks. After the coarse control, the lens seek control for moving a light beam to a target track position is carried out by controlling the velocity of the actuator. Independent from the lens-seek control, the positioner is controlled by a positioner servo circuit so as to keep the position of the actuator at the neutral position with respect to the positioner. Upon the light beam reaching the target track position, the control is switched to the fine control that is carried out according to a tracking error signal. At the same time with the control of the object lens on the actuator, a deviation and direction of the obJect lens from a neutral position with respect to the VCM positioner are detected by a position sensor, and the positioner is controlled by driving the voice-coil motor (VCM) to let the object lens always keep the neutral position with respect to the positioner. In this way, double servo control, i.e., the control of the moving part in the actuator and the control of the positioner, is achieved. The reason why the actuator is to be kept at the neutral position is because the roundness of a beam spot deteriorates to disable a read/write operation if the beam deviates from the neutral position. Namely, the read/write operation must be carried out only when the center of the light beam focuses on the target position.
In order not to respond rapidly to the mechanical play in the structure of the actuator, a servo frequency for the positioning control of the VCM positioner must be set very low. This causes a large tracking delay of the movement of the VCM positioner with respect to a servo operation of the actuator during a seek operation, so that it may take a long time after a light beam reaches a target track until the actuator is returned to the neutral position by the positioning control.
In this way, conventionally, a long settling time is needed after the completion of the lens seek operation until the read/write enable state is established, depending on a tracking delay of the positioner driven by the VCM. This hinders high-velocity accessing. It is therefore necessary to resolve this problem to realize high-velocity accessing.